THE SOURCES OF THE NTTKOGEN OF VEGETATION, ETC. 471 



matter in them, and its relations to the inorganic substances, entirely precludes the possi- 

 bility of our imitating, by artificial means, a natural soil, so as to include all its condi- 

 tions excepting a supply of combined Nitrogen. 



It is evident, therefore, that if all the conditions embraced in an ordinary soil were 

 essential to vegetable growth, the solution of the question of the assimilation of free 

 Nitrogen by plants would involve difficulties which our means of investigation in the 

 present state of science could not overcome. But the experiments to which attention 

 has been directed in the history of this subject, as well as others, the details of which 

 we shall give further on, show that such is not the case. They show that many of the 

 complicated conditions of an ordinary soil may be entirely dispensed with, so as to bring 

 the examination of it within our means of investigation, and yet to retain all the condi- 

 tions of healthy growth. 



In the experiments of the first year, 1857, two kinds of soil, or matrix, were used. 



One was prepared from an ordinary soil, so as more nearly to imitate the usual con- 

 ditions of vegetable growth. The other was prepared from volcanic pumice, with the 

 view to eliminate certain supposed sources of error which the prepared soil might intro- 

 duce. It was found, however, in the experiments of 1857, that there was no necessity 

 for this difference of matrix, and hence, in the experiments of 1858, only prepared soil 

 was used. 



The soil selected for the preparation of the matrix was a somewhat heavy one (clayey), 

 resting upon chalk, and interspersed with flints. The large stones were removed by 

 picking and sifting ; and the clayey lumps were powdered to prevent them from baking 

 into hard nodules during ignition. An attempt to ignite in ordinary clay crucibles was 

 not successful, owing to the reduction of the peroxide of iron to the state of black oxide, 

 and to the formation of sulphides from the reduction of the sulphates present, as indicated 

 by the vapours of sulphurous acid emitted during the ignition, and by the evolution of 

 sulphide of hydrogen on the addition of an acid to the mass after cooling. 



The combustion proceeded satisfactorily in a large cast-iron muffle, through which a 

 constant current of air could pass. The ignition was continued until a portion of the 

 soil assumed, on cooling, the red colour due to peroxide of iron, and exhibited no trace 

 of coaly matter. The mass thus prepared was taken from the muffle, and thrown into 

 a large vessel filled with distilled water. The water was rendered highly alkaline by 

 the quantity of caustic lime present. The fluid was decanted, and fresh portions of 

 water added several times during eight or ten days, until all the soluble matter was 

 removed. The residue was then dried, and retained for final ignition before being used. 

 The ferruginous and aluminous character of this soil-matrix pointed to the danger there 

 might be of its acting as a porous body, to promote the formation of nitrogenous com- 

 pounds independently of vegetable growth, on the one hand, or to absorb and retain the 

 ammonia given to the plant, or that which might be formed from the nitrogenous matter 

 of the seed, on the other. 



To ascertain the value of any influence exerted by the soil independently of the plant, 

 MDCCCLXI. 3 T 



